Monday, December 8, 2014

"I keep running into the same EMC problems over and over," EMC Engineer Kenneth Wyatt told an audience of some 40 engineers at a meeting of the Greater Boston IEEE EMC Society. The meeting took place on September 16 at the Bose Corp. headquarters in Framingham, Mass.

Gaps in return planes are one cause of the common-mode currents that produce unwanted emissions. Why? Because current returning to its source has to go around gaps, which lengthens the return path and enlarges the loop that current has to travel. In the video below, Wyatt explains where gaps occur, the emissions problems they cause, and what to do about them.

Wyatt then showed some examples of products that failed compliance tests and why they failed. Take LCD panels, a common problem. There are often gaps between the displays and their enclosures, which can let radiated energy escape. Figure 1, from Wyatt's presentation (download slides) illustrates the problem.

Wyatt explained why LCD displays can be a source of unwanted emissions because LED drivers use LVDS (low voltage differential signaling), which clocks the data at rates approaching 500 MHz and use fast edge speeds. These high frequency, fast edges, tend to couple common-mode currents onto the display cable and housing, which creates emissions that can escape from gaps between the display and enclosure. This is especially true as the length of the gap approaches a half wavelength of a harmonic frequency.

Figure 1. Unwanted emissions can escape from around LCD displays.

Poor bonding of cable through enclosures is another problem. Wyatt showed an industrial application where not only wasn't a cable shield connected to a return or reference location, but cables were routed through an enclosure, leaving gaps. Furthermore connectors used for other cables weren't properly connected. That is, connector shells didn't have solid connections to the enclosure. These and other issue can let emissions from clocks and digital circuits out or let outside emissions in.

Wyatt then demonstrated some of the concepts using a Rhode & Schwarz RTE 1104 oscilloscopewith FFT analysis and near-field probes. Figure 2, taken from Gaps in return planes - yes or no?, shows the test setup where he drove two transmission lines—one where the signal traveled over a gap in a return plane and another that didn't— with a digital pulse stream. The harmonics from the emissions were easy to see when the return trace passed over a gap.

Figure 2. A near-field probe and an oscilloscope using an FFT show how a gap in a return plane can create emissions when carrying digital signals.

To close the presentation, Wyatt showed some of the tools and equipment he uses to troubleshoot EMC problems, be they emissions or immunity. His toolbox included